Bacteria of the genus *Chromatium* are aquatic, strict anaerobes classified as purple sulfur Gammaproteobacteria. They use carbon dioxide ($$CO_2$$) as their sole carbon source, requiring sulfites as electron donors for survival and growth, and have served as a model for studying bacterial photosynthesis since the 1950s.

Purple Sulfur Bacteria, *Chromatium*

Purple sulfur bacteria are phototrophic bacteria that oxidize hydrogen sulfide ($$H_2S$$) into elemental sulfur and sulfuric acid. They obtain their purple color from the pigments bacteriochlorophylls and carotenoids.

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Phototrophic purple and green bacteria are classified based on their utilization of sulfur compounds for oxidation. Sulfur bacteria perform anoxygenic photosynthesis using sulfur compounds as electron donors, while nonsulfur bacteria use organic compounds, such as succinate and malate, as electron donors.

Classification of Purple and Green Bacteria

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Purple Sulfur Bacteria *Chromatium*

Green sulfur bacteria perform anoxygenic photosynthesis, using sulfites as electron donors to release free elemental sulfur, while utilizing sulfide ($$S^{2-}$$) to produce green bacteriochlorophyll.

Green sulfur bacteria

Green nonsulfur bacteria are phototrophic bacteria that are similar to green sulfur bacteria but use substrates other than sulfides, such as organic substrates or molecular hydrogen, as electron donors for oxidation.

Green non-sulfur bacteria

Purple nonsulfur bacteria are phototrophic bacteria that are similar to purple sulfur bacteria but use hydrogen instead of hydrogen sulfide for oxidation, utilizing organic substrates like succinate and malate as electron donors.

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